1. Field of the Invention
The present invention relates to circuitry for protecting equipment from electrical surges.
2. Description of the Related Art
Modern equipment which uses sophisticated micro-electronic circuitry is susceptible to electrical power disturbances. These disturbances come in different forms such as surges, spikes, noise, brownouts, and blackouts. Among these types of disturbances, blackouts occur least frequently. Brownouts occur more frequently. Blackouts and brownouts are the most expensive to protect against, requiring an uninterrupted power supply (UPS) to avoid. Surges, spikes, and noises occur frequently and can be protected against, to a considerable extent at reasonable cost, through the use of surge protectors. The present invention relates to such surge protectors.
Most equipment in use in the United States, such as computers, facsimile transmission machines, televisions, etc., is designed to operate at a voltage level of 120 volts rms. Voltage spikes above this level can damage or at least reduce the life of the equipment. The higher the voltage is above this level, the more damage it can cause. Therefore, the output voltage of a surge protection device should not be more than about 30% greater than the design voltage of the equipment or, in the case of standard 120 volt equipment, about 200 volts. Additionally, transient current spikes should be kept to a maximum of about 5 amperes.
In most existing surge protection devices, the load equipment is protected from a transient surge by diverting the surge to ground using clamping components such as crowbar circuits, metal oxide varistors (MOVs), gas discharge tubes and avalanche diodes and/or by blocking the surge from passing into protected equipment using a series blocking inductor or surge reactor.
With the former technique, if there is no good ground connection, very little protection is provided. This is because, if the high magnitude surge cannot find its way to ground, it may be diverted back into the supply. That is, a surge coming from the line, if not diverted to ground, may be diverted to the neutral conductor.
Second, many of such existing surge protection devices use MOVs as a means to divert incoming surges. While MOVs are very economical to use for this purpose, their performance degrades, typically dramatically, with each transient surge to which they are exposed. In normal use, depending on the number of transient surges, the lifetime of a MOV may be, for example, one to two years. Unless the user knows to replace such a surge protector regularly, the effectiveness of such a surge protector becomes limited over time. In most cases, there is no indication that the MOVs of such a device are degraded.
The latter technique of blocking rather than grounding the surge is usually effective in shielding the protected equipment from damage, but may result in damage to nearby unprotected equipment. Since the blocking inductor blocks but does not dissipate the incoming surge, the surge may be diverted. In most surge protection devices, the diversion of the blocked surge goes unnoticed. Thus, in many cases, devices not connected to the surge protector, but connected to the same power source, may be damaged.